1. Field of the Invention
The present invention relates to a degradable polymer composition and more particularly relates to a degradable polymer composition essentially consisting of a thermoplastic polymer composition primarily comprising polylactic acid and starch and/or a starch derivative. The thermoplastic and degradable polymer has recently drawn attention as a packaging material or a medical material.
2. Related Art of the Invention
The amount of plastics used for packaging has recently increased in view of beauty, sanitation, packing and transport of commodities. At the same time, a great amount of refuse discarded from households and factories has been increasing rapidly and causes a serious problem due to the shortage of landfill space in the periphery of large cities.
Conventional plastics for use in packaging are polyethylene, polypropylene, polystyrene, polyethylene terephthalate and polyvinyl chloride.
However, when these plastics are disposed in the natural environment, they remain without decomposition due to their stability and lead to damage in the scenery, pollution of the living environment for marine life and other various problems.
Many research and development activities have been carried out in order to prepare packaging materials from degradable polymers which do not generate these problems. For example, Japanese Patent Publications Sho 52-21530 and 52-42187 disclose techniques to provide degradability by blending non-degradable polymers, such as polyethylene, polypropylene and polystyrene with starch.
The resins prepared by these techniques collapse in shape by degradation of the starch. However, non-degradable polymer fragments remain intact and lead to environmental pollution.
On another hand, the amount of starch lower than the range causes a problem in that the original shape is retained after degradation of the starch portion, although film strength is decreased.
Polymers and copolymers of lactic acid have been known as thermoplastic and degradable polymers. The lactic acid polymer completely degrades in an animal body in the period of several months to a year. When these polymers are placed in a moist environment such as soil or sea water, the polymers start to decompose within several weeks and disappear in about a year. Decomposition products are lactic acid, carbon dioxide and water, all of which are harmless.
The raw material lactic acid can be prepared by fermentation of cheap materials such as corn starch and corn syrup, and can also be obtained from petrochemicals such as ethylene. U.S. Pat. Nos. 1,995,970, 2,362,511 and 2,683,136 disclose polymerization processes of lactic acid. U.S. Pat. Nos. 3,636,956 and 3,797,499 describe processes for preparing copolymers of lactic acid and glycolic acid. Lactic acid polymers are usually prepared from so-called lactide, a cyclic dimer of lactic acid. When glycolic acid is copolymerized, ring opening polymerization is carried out after mixing lactide with glycolide which is a dimer of glycolic acid. When the polymer is directly prepared by dehydrating condensation of lactic acid or a mixture of lactic acid and glycolic acid, a high molecular weight polymer cannot be obtained though reacted for a long time. On the other hand, straight chain polyester prepared by ring opening polymerization of lactide or a mixture of lactide and glycolide has a high molecular weight.
U.S. Pat. No. 4,333,919 discloses a formulation in which from 20 to 80% by weight of polyether antibiotics are incorporated in order to accelerate growth and feed efficiency in ruminants with a lactic acid-glycolic acid copolymer consisting of from 60 to 95% by weight of lactic acid and from 40 to 5% by weight of glycolic acid and having a weight average molecular weight of from 6,000 to 35,000. The patent describes that the formulation can be incorporated with auxiliary substances such as diluents, carriers, binders, excipients and other various adjuvants. Corn starch is included in these auxiliary substances.
The degradation rate of a polymer primarily comprising polylactic acid is almost determined by the hydrolysis rate of polylactic acid, and is relatively slow as compared with that of other microbiologically decomposable polymers.
Particularly in the case of polylactic acid having a high molecular weight, for example 50,000 or more, the degradation rate is remarkably slow and it is hence desired to find a method for accelerating the decomposition.
These polymers are usually processed into filaments or molded articles and utilized for biomedical slow-release materials, bone plates, screws and the above formulation for accelerating the growth and feed efficiency in ruminants. In these uses, the degradation rate in a living body is controlled by the addition of glycolic acid. However, an increased proportion of glycolic acid causes a change of polymer properties such as strength, hardness and elongation. Further, deterioration by moisture during use is accelerated and leads to problems.
Consequently, an object of the invention is to provide a degradable polymer composition which primarily comprises polylactic acid or a copolymer of lactic acid and hydroxycarboxylic acid, can be used for films, filaments and packaging materials, food packaging materials in particular, and has a suitable hardness and quick degradation rate.